Latest test, see my note in the shutdown section. Basically Orion froze and I had to force quit it, so this test should be considered moot as Orion wasn't given a chance to free memory before it was shut down gracefully.
All extensions disabled
Startup
Startup time: Sunday, August 07, 2022 at 12:22
Tabs currently open:
Processes running: 5
1 x "Orion" (83.1 MB)
1 x "Orion Web Content" (252.8 MB)
1 x "Orion Web Content (Prewarmed)" (12.0 MB)
1 x "Orion Networking" (13.6 MB)
1 x "com.apple.audio.SandBoxHelper" (2.8 MB)
Total memory usage: 364.3 MB
Shutdown
Note: Orion completely froze up and I had to force kill it from the activity monitor. I took a screenshot of the current values in the activity monitor, however I don't have full details on the tabs I had open at the time. Unfortunately I'd suggest this test be moot, given that I wasn't able to carefully "walk down" the amount of tabs open, etc, which should give Orion a chance to free some memory it was holding on to. However, I included it in the interest of full disclosure.
Shutdown time: August 10, 2022 at 16:22
Tabs currently open: approx 20 across 2 windows
Processes running: 31
1 x "Orion" (553.8 MB)
22 x "Orion Web Content (Prewarmed)" (744.8, 737.1, 539.2, 456.6, 454.8, 365.6, 361.7, 336.9, 290.8, 287.2, 277.8, 263.6, 235.0, 206.6, 193.0, 190.1, 169.1, 143.3, 129.6, 118.3, 106.7, 12.4)
3 x "Orion Web Content" (712.5, 685.3, 143.1)
1 x "com.apple.WebKit.GPU.Orion" (181.2)
1 x "Orion Networking" (163.3)
1 x "VTDecoderXPCService" (61.9)
2 x "MTLCompilerService" (6.0, 5.4)
Total memory usage: 9,132.7 MB

Going to test Orion again for a couple of days without Bitwarden, then will test Safari to compare.